A conventional activated sludge wastewater treatment system has a primary clarifier followed by one or more tanks in which mixed liquor is maintained under aerobic, anoxic or anaerobic conditions. Mixed liquor leaving these tanks is separated in a second clarifier to produce an effluent and activated sludge. Some of the activated sludge is returned to the process tanks. In some plants, the remainder of the activated sludge is thickened and then sent to an anaerobic digester with sludge from the primary clarifier. In some cases, the second clarifier is replaced with a membrane separation unit to produce a membrane bioreactor (MBR).
US publication 2013/0134089 A1 describes a wastewater treatment process that uses enhanced primary treatment to remove suspended solids from raw wastewater. Primary sludge is treated in a fermenter. Primary effluent is treated by biological nutrient removal to produce a treated effluent and waste activated sludge (WAS). The WAS is treated in an anaerobic digester, which also treats sludge from the fermenter.
In a membrane biofilm reactor (MBfR), a membrane is used both to support a biofilm and to transport a gas to the biofilm. Membrane-aerated biofilm reactors (MABR) are a subset of MBfRs in which an oxygen-containing gas is used. U.S. Pat. No. 7,169,295 describes a membrane supported biofilm film reactor with modules having fine hollow fiber membranes used in tows or formed into a fabric.